RU2186831C2 - Hydrodesulfurization method and method for improving quality of hydrocarbon stock - Google Patents

Abstract

FIELD: petrochemical processes. SUBSTANCE: low-sulfur gasoline is obtained from olefin sulfur-containing cracking ligroin (naphtha) by treatment with acid catalyst, preferably zeolite with middle pore size such as ZSM-5 for cracking of low-octane paraffins and olefins under rather mild conditions at limited aromatization of olefins and naphthenes. Benzene-enriched complex raw material together with ligroin is subjected to alkylation in order to reduce level of benzene. The first stage is followed by hydrodesulfurization using hydrofining catalyst such as CoMo/alumina. EFFECT: reduced sulfur and benzene levels. 10 cl, 5 tbl

Description

 The invention relates to a method for improving the quality of hydrocarbons. More specifically, it relates to a method for improving the quality of petroleum fractions with boiling ranges of gasoline containing significant proportions of benzene and sulfur impurities, while minimizing the octane losses that occur after sulfur removal by hydrogenation.

 Catalytic cracked gasoline makes up the bulk of gasoline products in the United States. When the cracked feed contains sulfur, the products of the cracked process usually contain sulfur impurities, which usually need to be removed usually by hydrotreating in order to bring them into line with the standard standards for the product in question. It is believed that in the future these standard norms will become more stringent, allowing no more than 300 wt. ppm sulfur (or even lower) in gasoline and other fuels. Despite the fact that the sulfur content in the product can be reduced by hydrodesulphurization of the cracked feed, this process is expensive both from the point of view of capital construction and from the point of view of operating costs, since significant amounts of hydrogen are consumed.

 As an alternative to desulfurization of the cracked feed, products that need to be brought into compliance with standard low sulfur standards can be hydrotreated, usually using a catalyst containing a Group VIII or VI element of a periodic system of elements such as cobalt or molybdenum, or individually or in combination with each other, on an appropriate carrier, such as alumina. In the hydrotreatment process, molecules containing sulfur atoms are mildly hydrocracked to convert sulfur to an inorganic form, hydrogen sulfide, which can be removed from the liquid hydrocarbon product in a separator. Although this hydrotreating method is an efficient process that has been put into practice for many years for gasoline and heavier oil fractions in order to obtain satisfactory products, it has disadvantages.

 Cracking naphtha (naphtha cracking) originating from a catalytic cracking unit and without any further processing such as purification operations has a relatively high octane number due to the presence of olefin components, and essentially cracked gasoline is an excellent contributor to the gas octane range. It contributes to the production of a large quantity of product with a high octane mixing ratio. In some cases, this fraction can produce up to half of gasoline in the aggregate of refinery products.

 Other highly unsaturated fractions boiling within the boiling range of gasoline, which are obtained at some refineries or petrochemical plants, include pyrolysis gasoline, obtained as a by-product from the cracking of oil fractions to form light olefins, mainly ethylene and propylene.

 Pyrolysis gasoline has a very high octane number, but is completely unstable in the absence of hydrotreating, since, in addition, the desired olefins boiling within the boiling range of gasoline also contain a significant proportion of diolefins, which tend to form resins during storage.

 Hydrotreating these sulfur-containing cracking-naphtha fractions (naphtha cracking fractions), as a rule, causes a decrease in the olefin content and, consequently, a decrease in the octane number; when the degree of desulfurization increases, the octane number of the product with the boiling range of gasoline decreases. A certain amount of hydrogen can also cause hydrocracking to some extent, as well as olefin saturation, depending on the conditions of the hydrotreating operation.

 Various proposals have been put forward for the removal of sulfur while preserving olefins, which make a positive contribution to the octane number. Sulfur impurities tend to be concentrated in the heavy gasoline fraction, as noted in US Pat. No. 3,957,625 (Orkin), which proposes a method for removing sulfur by hydrodesulfurization of a heavy fraction obtained by catalytic cracking of gasoline in order to preserve the octane contribution from olefins, which are mainly in the lighter fraction. According to one type of conventional commercial operation, the heavy gasoline fraction is purified in this way. Alternatively, the hydrodesulfurization selectivity relative to olefin saturation can be changed by selecting a suitable catalyst, for example, by using magnesium oxide as a carrier instead of the more common alumina. US Pat. No. 1,049,542 (Gibson) proposes a process in which a copper catalyst is used to desulphurize an olefinic hydrocarbon feed as well as obtained by catalytic cracking of light naphtha.

 In any case, not paying attention to the mechanism by which this occurs, the decrease in the octane number, which takes place as a result of sulfur removal by hydrotreating, causes a dissonance between the increasing need to produce gasoline with a higher octane number and the need to obtain completely combustible, less polluted low-sulfur fuels. This inherent dissonance is even more noticeable in the current situation with low-sulfur oils.

 Other purification methods for catalytic cracked gasolines have also been proposed in the past. For example, US Pat. No. 3,759,921 (Brennan) describes a method for improving the quality of gasoline obtained by catalytic cracking by separating it into a heavier and lighter fraction and purifying the heavier fraction using a ZSM-5 catalyst, after which the purified fraction is mixed with the lighter fraction. Another method in which cracked gasoline is separated prior to purification is described in US Pat. No. 4,062,762 (Howard), which discloses a method for the desulfurization of naphtha by separating naphtha into three fractions, each of which is subjected to desulfurization by different methods, after which the fractions are combined.

US Pat. No. 5,143,596 (Maxwell) and European Patent No. 420326-B1 describe methods for refining a sulfur-containing feedstock in the gasoline industry by reforming using a sulfur-resistant catalyst that is aromatically selective. Catalysts of this kind include metal-containing crystalline silicates, including zeolites, such as gallium-containing ZSM-5. The process described in US Pat. No. 5,143,596 relates to hydrotreating an aromatic effluent from a reforming step. The conversion of naphthenes and olefins to aromatic hydrocarbons is at least 50% under the harsh conditions used, typically at temperatures of at least 400 ^° C (750 ^° F) and usually higher, for example at 500 ^° C (930 ^° F). Under such conditions, conventional reforming is typically accompanied by significant and undesirable yield losses, usually more than 25%, and the same is true for the methods described in these publications: C ₅₊ yields in the range of 50-85% are reported in European Patent 420326. the method, therefore, has a traditional disadvantage of reforming, so that the problem of developing a method in which it is possible to reduce the sulfur content in cracking naphtha while minimizing yield losses, as well as reducing hydrogen consumption, remains in the same state.

 US Pat. No. 5,346,609 describes a method for reducing the sulfur content of cracked naphtha, first by hydrotreating the naphtha to convert sulfur to an inorganic form, then by treating it with a catalyst such as ZSM-5 to recover the octane loss during the hydrotreating step, mainly by selective cracking of low octane paraffins. This method, which is commercially successful, leads to a low-sulfur naphtha with a good yield, which can be directly introduced into the gasoline collection.

 Another aspect of recent binding regulations is the need to reduce the benzene, a suspected carcinogen, in gasoline.

 Benzene is found in numerous light refinery streams that mix in the refinery's gasoline collection, mainly in the reforming product, which is desirable as a component in the gasoline collection due to its high octane number and low sulfur content. The relatively high content of benzene in it, however, necessitates further purification in order to fulfill the requirements of binding regulations. Various methods have been proposed for reducing benzene in refinery streams, such as, for example, fluidized-bed processes described in US Pat. Nos. 4,827,069, 4,950,387 and 4,992,607, in which case benzene is converted to alkyl aromatic hydrocarbons by alkylation with light olefins. Benzene can be derived from cracked naphtha or from benzene-rich streams such as reformate products. Similar methods in which the removal of benzene is accompanied by reductions in sulfur content are described in applications for US patents 08/286894 (Mobil Case 6994FC) and 08/322466 (Mobil Case 6951FC) and in US patent 5391288. A method of reducing benzene in light refinery streams, such as a reformate and light fluid catalytic cracking gasoline, by alkylation and transalkylation together with heavy alkyl aromatic hydrocarbons, is described in US Pat. No. 5,347,061.

SUMMARY OF THE INVENTION
Currently, the authors have developed a method for the catalytic desulfurization of cracked fractions with a boiling point of benzene, which allows to reduce the sulfur content to acceptable levels essentially without decreasing the octane number. At the same time, the present method allows to reduce the benzene content in light refinery streams, such as a reformate. The usefulness of this method is to reduce the consumption of hydrogen and to reduce the formation of mercaptans in comparison with the method described in US patent 5346609, as well as the potential to reduce the content of benzene in other streams.

 According to the present invention, a method for improving the quality of cracked naphtha includes a first catalytic treatment step in which a cracked naphtha feed is treated in conjunction with a light benzene-containing hydrocarbon stream to convert benzene, olefins and some paraffins to a combined feed using a zeolite or other acid catalyst. The reactions that take place are mainly the selective cracking of low-octane paraffins and olefins and the alkylation reactions in which benzene is converted into alkyl aromatic hydrocarbons. Many of them increase the octane content in cracking naphtha and greatly reduce the content of olefins in it, which, in turn, reduce hydrogen consumption and reduce octane losses during the subsequent hydrodesulfurization stage. The degree of aromatization of olefins and naphthenes is limited due to the mild conditions used in the processing using an acid catalyst; the aromatic hydrocarbon content in the final hydrotreatment product may in some cases be lower than that in the combined feed.

In its usual form, in practice, the method includes, in the first stage, introducing into contact the raw materials (sulfur-containing cracking-naphtha fraction and the benzene-rich combined reforming products) with a solid acid zeolite catalyst with an average pore size at a temperature of 177-427 ^o С (350-800 ^o F), a pressure of 2172-6998 kPa (300-1000 psi), a fluid hourly space velocity of 1-6 and a hydrogen to hydrocarbon ratio of 180-445 l / l (under normal conditions) [1000-2500 standard cubic feet per barrel of feed ] for the alkylation of benzene in a plant olefins with the formation of alkyl aromatic hydrocarbons and for cracking olefins and low-octane paraffins in the feed, with the conversion of olefins and naphthenes to aromatic hydrocarbons, flowing to contents of less than 25 wt. %, and the conversion of benzene (to alkyl aromatic hydrocarbons) 10-60%. The intermediate product is then subjected to hydrodesulfurization in the presence of a hydrodesulfurization catalyst at a temperature of 260-427 ^o C (500-800 ^o F), a pressure of 2172-6998 kPa (300-1000 psi), a fluid hourly space velocity of 1-6 and a ratio of hydrogen to hydrocarbon feed 180 -445 l / l (under normal conditions) [1000-2500 standard cubic feet per barrel of raw material], for converting sulfur-containing compounds into an intermediate inorganic sulfur product and obtaining a desulfurized product with a total yield of liquid products of at least 90 vol. %

 Compared to the purification result described in US Pat. No. 5,346,069, where the cracked ligroin is first subjected to hydrodesulfurization followed by treatment with an acid catalyst such as ZSM-5, the present process works with a reduced hydrogen consumption due to olefins removed in advance. Also, by performing hydrodesulfurization after the initial treatment, the formation of mercaptans is eliminated by combining hydrogen sulfide with an olefin using a zeolite catalyst, which potentially leads to a higher degree of desulfurization or to reduce the need for processing of a further product, for example, as described in U.S. patent application 08/001681.

 The method can be used to desulphurize light and broad ligroin fractions while maintaining the octane number so as to eliminate the need for reforming such fractions or at least without the need to reform such fractions to the previously considered necessary degree.

 In practice, it may be desirable to hydrotreat the cracked ligroin before it is brought into contact with the catalyst in the first aromatization / cracking step in order to reduce the diene content of the ligroin and thus increase the catalyst life. Only a very limited degree of saturation of the olefin is achieved in the pre-cleaner and only a small amount is subjected to desulfurization during this time.

Feedstock
One type of feedstock for the method includes a sulfur-containing oil fraction, which boils within the boiling range of gasoline. Raw materials of this type typically include light naphtha, typically having a boiling range of C ₆ to 166 ^° C (330 ^° F), a broad naphtha fraction, usually having a boiling range of C ₅ to 216 ^° C (420 ^° F), and heavier naphtha boiling within 127-211 ^o C (260-412 ^o F), or heavy gasoline fractions boiling, or at least boiling, within 166-211 ^o C (330-500 ^o F), preferably 166-260 ^o C (330-412 ^° F). In many cases, the feed has 95% of the temperatures (determined according to ASTM D 86), at least 163 ^° C (325 ^° F), and preferably at least 177 ^° C (350 ^° F), for example 95% of the temperatures are at least 193 ^o C (380 ^o F) or at least 220 ^o C (400 ^o F).

 Catalytic cracking is a suitable source of cracking naphtha, usually fluid catalytic, however, thermal cracking methods such as coking can also be used to produce usable feedstocks such as ligroin, pyrolysis gasoline, and others from thermal cracking of ligroins.

The method can be carried out using a pure ligroin fraction obtained from the catalytic or thermal cracking step, or, alternatively, a part thereof. Since sulfur tends to concentrate in higher boiling fractions, it is preferable, especially when the plant capacity is limited, higher boiling fractions are separated and subjected to the steps of the present process without treating the lower boiling fraction. The boiling boundaries between the treated and untreated fractions may vary according to the presence of sulfur compounds, however, the boiling boundaries of the fractions in the range of 38 ^{° C.} (100 ^° F.) to 150 ^{° C.} (300 ^° F.) are generally suitable, more typically in the range from 93 ^o C (200 ^o F) to 150 ^o C (300 ^o F). The exact choice of the boiling range of the fractions depends on the standard norms for sulfur for gasoline, as well as on the type of sulfur compounds present: lower boiling ranges are usually necessary in the case of standard norms for lower sulfur content in the product. Sulfur present in components boiling below 65 ^° C (150 ^° F) is usually in the form of mercaptans, which can be removed by treatment with an extractive type such as Merox treatment, however hydrotreating is suitable for removing thiophene and other cyclic compounds sulfur present in higher boiling components, for example, in fractions of components boiling above 82 ^o C (180 ^o F). Processing a lower boiling fraction in an extractive type process combined with hydrotreating higher boiling components can therefore be an economically preferred process parameter. Broader boiling ranges of the fractions are preferable to minimize the amount of feed that passes through the hydrotreatment unit, and ultimately select the boiling ranges of the fractions along with other process parameters, such as the type of extraction, therefore, desulfurization is carried out in accordance with standard product standards, feedstock limitations and other factors.

The sulfur content in the cracked fraction depends on the sulfur content in the cracking feedstock, as well as the boiling range of the selected fraction used as feedstock in the method. Lighter fractions, for example, tend to have lower sulfur contents than higher boiling fractions. Practically, the sulfur content exceeds 50 wt. ppm and usually is more than 100 wt. ppm, in most cases, more than 500 wt. ppm For fractions that have 95% temperatures above 193 ^o C (380 ^o F), the sulfur content may exceed 1000 wt. ppm and can be as high as 4000 or 5000 wt. ppm or even higher as shown below. The nitrogen content is not a characteristic of the raw material, as the sulfur content, and is preferably not more than 20 wt. ppm, although higher amounts of nitrogen, usually up to 50 wt. ppm can be found in some higher boiling feedstock with 95% temperatures above 193 ^° C (380 ^° F). The nitrogen content, however, is usually not higher than 250 or 300 wt. ppm As a result of the cracking that precedes the steps of the present process, the feed for the hydrodesulfurization step is olefin with an olefin content of at least 5 wt.% And more typically in the range of 10-20 wt.%, For example 15-20 wt.%; preferably, the feed contains olefins in an amount of 10-20 wt.%, sulfur in an amount of 100-5000 wt. ppm, nitrogen in an amount of 5-250 wt. ppm and benzene in at least 5 vol.%. Thermal cracking ligroins often contain dienes, however, as described below, they are preferably removed by hydrogenation as a pre-treatment step.

The combined feed for the method includes a light fraction boiling within the boiling range of gasoline, in which there is a relatively high content of aromatic hydrocarbons, mainly benzene. The benzene-enriched feed typically contains at least 5 vol.% Benzene, in particular 20-60 vol.% Benzene. A specific refinery source of the fraction is the reforming product fraction. The fraction contains smaller amounts of lighter hydrocarbons, usually less than 10% C ₅ and lower hydrocarbons, and minor amounts of heavier hydrocarbons, usually less than 15% C ₁ and higher hydrocarbons. This reformed combination feed usually contains very small amounts of sulfur, as it is usually desulfurized before reforming.

Examples include a reforming product from a reformer with a fixed oscillating or moving catalyst bed. The most suitable fraction of reforming products is the middle reforming fraction, that is, the reforming product from which the lightest and heaviest parts are removed by distillation. This is preferably a reforming product with narrow boiling ranges, i.e. a C ₆ or C ₅ / C ₇ fraction. This fraction can be obtained in the form of a complex mixture of hydrocarbons obtained in the form of an overhead stream to a hexano-distillation column from a pentano-distillation column. The composition varies in a wide range, depending on the number of factors, including the rigidity of the process in the reforming installation and the raw materials for the reforming installation. From these streams, C ₅ , C ₄ hydrocarbons and lower hydrocarbons are usually removed in a pentano-distillation column and a butano-distillation column. Therefore, usually the middle fraction of the reformate product contains at least 70 wt.% C ₆ hydrocarbons and preferably at least 90 wt.% C ₆ hydrocarbons. Other sources of benzene-rich feeds include light naphtha obtained from the coking process of naphtha or pyrolysis gasoline.

With respect to the boiling range, these benzene-enriched fractions can be determined from a final boiling point of 121 ^° C (250 ^° F) and preferably not higher than 110 ^° C (230 ^° F). Preferably, the boiling range is from 38 ^{° C.} (100 ^° F.) to 180 ^{° C.} (212 ^° F.) and more preferably from 66 ^{° C.} (350 ^° F.) to 93 ^{° C.} (200 ^° F.) and even more preferably from 71 ^o C to 93 ^o C (from 160 ^o F to 200 ^o F).

In the table. 1 shows the properties of a suitable middle fraction of a C ₆ -C ₇ reforming product with a (final) boiling point of 121 ^° C (250 ^° F).

 In the table. 2 shows the properties of the more preferred benzene-rich middle fraction of the product, which is more paraffin.

Method Description
The selected sulfur-containing feedstock with boiling range of gasoline together with the benzene-enriched combined feed is processed in two stages by first passing ligroin with the combined feed through a selective acid catalyst. At this stage, the olefins present in the cracking naphtha alkylate benzene and other aromatic hydrocarbons to form alkyl aromatic hydrocarbons, while at the same time, the amount of olefins increases due to the selective cracking of low-octane olefin paraffins from components of one or both types of raw materials.

Olefins and naphthenes can be converted to aromatic hydrocarbons, however, the degree of aromatization is limited due to the relatively mild conditions, mainly the temperatures used in this step of the process. The effluent from this stage then enters the hydrotreating stage, in which the sulfur compounds present in the ligroin feedstock, which are mostly unconverted in the first stage, are converted into an inorganic form (H ₂ S), which can be removed in a separator following hydrodesulfurization. Since no products are obtained in the first processing stage using an acid catalyst that interfere with the operation of the second stage, the effluent of the first stage can be directly sent to the second stage without the need for separation between the stages.

 The particle size and nature of the catalysts used in both stages are usually determined by the type of process used, such as a liquid phase process with a fixed catalyst bed and a downward flow of feed; a method with a flowing thin stream of phase with a fixed catalyst bed and an upward flow of raw materials; a fluidized bed bubbler method; or a transport method with a fluidized bed of catalyst. All of these various process schemes are well known, although a fixed catalyst bed and a downward flow of feed is preferred because of its ease of use.

First stage processing
Combined feeds are first treated by contacting with an acid catalyst under conditions under which the benzene is alkylated with olefins to form alkyl aromatic hydrocarbons. Most of the benzene comes from a combined feed, for example from a reformate, although some aromatization of the olefins found in the naphtha may occur, with the formation of additional benzene. Under mild conditions, especially with respect to the temperature used in this step, a very high degree of aromatization of olefins and naphthenes is usually prevented. As a rule, the conversion of olefins and naphthenes to new aromatic hydrocarbons is not more than 25 wt. %), and usually lower, typically not more than 20 wt.%. Under very mild conditions in the first stage, the total content of aromatic hydrocarbons in the final hydrotreatment product may actually be lower than that in the combined feed, due to the somewhat hydrogenation of aromatic hydrocarbons in the second stage of the reaction.

 Selective cracking of low-octane paraffins, mainly n-paraffins, and olefins leads to an increase in the octane number of the product with increasing formation of olefins, which may also result from the alkylation of aromatic hydrocarbons, mainly benzene. These reactions proceed under relatively mild conditions and yield losses are low. During both stages of the process, the total yields of liquid products are typically at least 90 vol.% And may be higher, for example, 95 vol. % In some cases, the yield of liquid products may be above 100 vol. % due to expansion of volume due to ongoing reactions.

 Compositionally, the first processing step is the selective cracking of low-octane feed components in combination with the alkylation of aromatic hydrocarbons. Olefins come from raw materials, as well as from an increasing amount of paraffins and olefins when cracking combined feeds. Certain isomerization of n-paraffins into branched chain paraffins with an increase in the octane number may occur, which further contributes to the octane number of the final product. Amounts of benzene decrease with increasing degree of alkylation at higher temperatures of the first stage, and the conversion of benzene is typically in the range of 10-60%, more typically 20-50%.

The conditions used in this step of the process are favorable to these reactions. Typically, the temperature of the first stage is 150-455 ^o C (300-850 ^o F), preferably 177-425 ^o C (350-800 ^o F). The pressure in this reaction zone is not critical since hydrogenation does not occur, although lower pressure in this stage favors the favorable formation of olefins by cracking the low-octane components of the feed stream. The pressure, which therefore mainly depends on the suitability for carrying out this stage, is usually 445-10445 kPa (50-1500 psi), preferably 2170-7000 kPa (300-1000 psi), at hourly volumetric flow rates of the liquid, usually 0.5 -10 (hour ^-1 ), usually 1-6 (hour ^-1 ). The ratio of hydrogen to hydrocarbon feed is typically 0-890 L / L (under normal conditions) [0-5000 standard cubic feet per barrel], preferably 18-445 L / L (under normal conditions) [100-2500 standard cubic feet per barrel ] to minimize catalyst aging.

In the first stage, usually there is a change in the volume of material with the boiling range of gasoline. A slight decrease in the volume of the liquid product occurs as a result of the conversion to lower boiling products (C _5- ), however, the conversion to C _6- products is typically not more than 10 vol.% And usually below 5 vol.%. A further decrease in volume usually occurs due to the conversion of olefins to aromatic hydrocarbons or their incorporation into aromatic hydrocarbons, but with limited aromatization, which is usually negligible. If the feed contains significant amounts of higher boiling components, the content of C _5- products may be relatively lower and, therefore, the use of higher boiling ligroins is favorable, especially fractions with 95% temperatures above 177 ^° C (350 ^° F) and even more preferably higher 193 ^o C (380 ^o F) or, for example, above 205 ^o C (400 ^o F). Typically, however, 95% of the temperatures do not exceed 270 ^o C (520 ^o F) and usually no higher than 260 ^o C (500 ^o F).

 The catalyst used in the first step of the process has sufficient acid functionality to carry out the desired cracking, aromatization, and alkylation reactions. For this purpose, it has a significant degree of acid activity, and for this purpose, the most preferred materials are solid, crystalline catalytic materials such as molecular sieves, and these solid materials have an average pore size and topology behaving like a zeolite material, which in the aluminosilicate form has a permeability index of 2 -12. Preferred catalysts for this purpose are catalytic materials behaving as medium pore zeolites, for example acting as acid materials having a top pore aluminosilicate zeolite topology. These zeolite catalyst materials are, for example, those which, in their aluminosilicate form, have a permeability index of 2-12. Reference should be made to US Pat. No. 4,784,745 with respect to determining a permeability index and describing a method for measuring this value, as well as the usefulness of the detailed information provided for a number of catalytic materials having the appropriate topology and structure of the pore system.

 Preferred medium pore aluminosilicate zeolites have the topology SZM-5, ZSM-11, ZSM-12, ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50 or MCM-22, MSM -36, MSM-49 and MSM-56, preferably in aluminosilicate form. (Modern catalytic materials identified by MSM numbers are disclosed in the following patents: MSM-22 zeolite is described in US Pat. No. 4,954,325; MSM-36 zeolite is described in US Pat. No. 5,250,277 and 5,292,698; MSM-49 zeolite is described in US Pat. 56 is described in US Pat. No. 5,362,697). However, other catalytic materials having appropriate acid functionality may be used. A particular class of catalytic materials that can be used are, for example, large pore zeolites that have a permeability index of up to 2 (in aluminosilicate form). Zeolites of this type include mordenite, beta zeolite, fayazitov, as well as zeolite Y and ZSM-4. Other refractory solid materials having the desired acid activity, pore structure and topology may also be used.

The catalyst should have sufficient acid activity to convert the corresponding components of the ligroin feed as described above. The only measure of the acid activity of a catalyst is its alpha number. Alpha test is described in US patent 3354078 and in J. Catalysis, 4, 527 (1965); 6,278 (1966); and 61, 395 (1980). The experimental conditions of the test used to determine the alpha values given in this specification include a constant temperature of 538 ^{° C.} and a variable volumetric flow rate, as described in detail in J. Catalysis, 61, 395 (1980). The catalyst used in this step of the process accordingly has an alpha activity of at least 20, typically in the range of 20-800, and preferably at least 50-200. This catalyst does not require very high acid activity, so it is desirable only for cracking and rearrangement of such an amount of naphtha that is necessary to maintain the octane number without greatly reducing the volume of the product beyond the boiling range of gasoline.

 The active component of the catalyst, such as zeolite, is usually used in combination with a binder or carrier, since the particle sizes of the behaving as pure zeolite material are too small and lead to an excessive decrease in pressure in the catalytic layer. This binder or carrier, which is preferably used to carry out this step, is a suitable and refractory binder material. Examples of such materials are well known and typically include silica, silica-alumina, silica-zirconia, silica-titania, alumina.

 The catalyst used in this step of the process may not contain any metal component for hydrogenation, or it may include the function of a metal for hydrogenation. If it is found that it is desirable to use specific raw materials under existing conditions, metals such as ordinary Group VIII metals, mainly molybdenum, or combinations thereof are usually used. Noble metals, such as platinum or palladium, do not have an advantage over nickel or other common metals.

Hydrotreating in the second stage
Hydrotreating the effluent of the first stage can be accomplished by contacting the feed with a hydrotreating catalyst. Under hydrotreating conditions, at least some of the sulfur compounds present in naphtha that remain unchanged after the cracking / aromatization step are converted to hydrogen sulfide, which is removed when the desulfurized effluent enters the separator following the hydrotreatment unit. The desulfurized product boils essentially within the same boiling range as the raw material (gas boiling range), but has a lower sulfur content than the raw material. The sulfur content in the product is typically below 300 wt. ppm and in most cases below 50 wt. ppm The nitrogen content is also reduced to levels typically below 50 wt. ppm, usually below 10 wt. ppm by conversion to ammonia, which is also removed in the separation step.

 If the pre-treatment step is carried out before the catalytic treatment in the first step, the same type of hydrotreating catalyst can be used as in the second step of the process, but the conditions can be milder so as to minimize olefin saturation and hydrogen consumption. Since the saturation of the first double bond in dienes is kinetically / thermodynamically favorable relative to the saturation of the second double bond, this goal can be achieved by appropriate selection of conditions. Appropriate combinations of processing parameters, such as temperature, hydrogen pressure and mainly space velocity, can be found empirically. The effluent from the pre-treatment plant can be directly sent to the first stage of the treatment, with hydrogenation reactions proceeding with a slightly exothermic result, providing a useful temperature increase to initiate for the most part endothermic reactions during the processing in the first stage.

In order to preserve the octane number of the product and the volume, the conversion to products boiling below the boiling range of gasoline (C _5- ) is kept to a minimum during the second stage of desulfurization. The temperature of this step is suitably 220-454 ^° C (400-850 ^° F), preferably 260-400 ^° C (500-750 ^° F), the exact choice being dependent on the desulfurization required to produce the feed using the selected catalyst. The temperature increase occurs under exothermic reactions, with values of 11-55 ^o C (20-100 ^o F) are typical for most conditions and the temperature at the inlet to the reactor is preferably in the range 260-400 ^o C (500-750 ^o F).

Since cracking naphtha desulfurization is usually easy, low to medium pressures can be used, typically 445-10443 kPa (50-1500 psi), preferably 2170-7000 kPa (300-1000 psi). The pressures represent the total pressure of the system at the inlet to the reactor. The pressure is usually chosen so as to maintain the desired aging rate of the catalyst in use. The volumetric hourly rate of the liquid (desulfurization step) is typically 0.5-10 (hour ^-1 ), preferably 1-6 (hour ^-1 ). The ratio of hydrogen to hydrocarbon feed is typically 90-900 l / l (under normal conditions) [500-5000 standard cubic feet per barrel], typically 180-445 l / l (under normal conditions) [1000-2500 standard cubic feet per barrel ], usually 180-445 l / l (under normal conditions) [1000-2500 standard cubic feet per barrel]. The degree of desulfurization depends on the sulfur content in the feed and, naturally, on standard sulfur standards in the product with appropriately selected reaction parameters. Typically, according to the method, a combination of conditions is used, such that desulphurization should be at least 50%, preferably at least 75%, compared to the sulfur content of the feed. It is not necessary to achieve very low amounts of nitrogen, however, low nitrogen contents can increase the activity of the catalyst in the second stage of the process. Typically, the denitrogenation that accompanies desulfurization results in an acceptable organic nitrogen content in the feed in the second stage of the process.

 The catalyst used in the hydrodesulfurization step corresponds to a traditional desulfurization catalyst, which is a metal of group VI and / or group VIII of the periodic system of elements on an appropriate support.

 Group VIII metal is usually molybdenum or tungsten, and Group VI metal is usually nickel or cobalt. Typical combinations are nickel-molybdenum or cobalt-molybdenum. Other metals having hydrogenation functionality are also suitable for this step. The catalyst carrier is usually a porous solid, usually alumina or silica-alumina, however, other porous substances, such as magnesium oxide, titanium dioxide or silica, used either individually or mixed with oxide, can also be used. aluminum or silica-alumina.

 Particle size and the nature of the catalyst are usually determined by the type of conversion process being carried out, such as a liquid phase process with a fixed catalyst bed and a downward flow of feed; liquid-phase method with a fixed catalyst bed and with an upward flow of raw materials; a liquid-phase or gas-phase bubble-type method with a fixed bed of fluidized material; or a liquid-phase or gas-phase method of a transport type with a fluidized bed of catalyst, as described above, moreover, it is preferable to operate in a type with a downward flow of raw materials and a fixed catalyst bed.

Information confirming the possibility of carrying out the invention
Examples
Three volume parts of the fraction with a boiling point of 99 ^° C + (216 ^° F +) of fluid catalytic cracking naphtha are mixed with one part of the middle fraction of the reforming product to obtain a combined feed with the composition and properties indicated in the table. 3. Combined feedstock is introduced together with hydrogen into a fixed-bed reactor containing a ZSM-5 catalyst with the properties shown in table. 4.

All effluent from the first reactor is sent to the second fixed bed reactor containing a commercially available CoMo / Al ₂ O ₃ catalyst (Akzo K742-3Q). The feed rate is constant, such that the hourly space velocity of the liquid through the ZSM-5 catalyst is 1.0 (hour ^-1 ) and the hourly space velocity of the liquid through the hydrotreatment catalyst is 2.0 (hour ^-1 ). The total pressure in the reactor is maintained at 4171 kPa (590 psi) and the simultaneous supply of hydrogen is constant at 356 l / l (under normal conditions) [2000 standard cubic feet per barrel] of naphtha. The temperature of the ZSM-5 reactor varies from 205 ^° C to 427 ^° C (400 ^° F to 800 ^° F), while the temperature of the hydrotreating reactor (HDT) is 260-370 ^° C (500-700 ^° F). The results are presented in table. 5.

As can be seen from the table. 5, an increase in temperature in the case of ZSM-5 at a constant hydrotreatment temperature strictly leads to an increase in octane numbers and a decrease in C ₅₊ yields. Significant benzene conversions of about 40% are also observed at ZSM-5 temperatures of 388 ^° C to 427 ^° C (750 ^° F to 800 ^° F) compared to 13%, due to saturation in the case of using a hydrotreating catalyst. You can also achieve degrees of desulfurization above 94%. Hydrogen consumption increases with increasing temperature of ZSM-5 due to the increasing conversion of cracking naphtha olefins when using an acid catalyst rather than in the case of hydrogen-consuming reactions using a hydrotreating catalyst; hydrogen consumption can be further reduced by lowering the hydrotreating temperature to 260 ^{° C.} (500 ^° F.) with a slight hydrodesulfurization effect. This lower hydrotreating temperature also leads to an increase in the octane number of the product, while the saturation of aromatic hydrocarbons decreases. Aromatization of olefins and naphthenes of the feedstock occurs to a low degree and during both stages of the process, the content of aromatic hydrocarbons can even be reduced relative to the feedstock. The yield of liquid products is high in all cases, and the highest yield is observed at a low temperature of the first stage, which provides an increase in the volume of the product.

Claims (10)

1. The method of hydrodesulphurization of a combined hydrocarbon feedstock, including fractions containing sulfur, olefins and benzene, and reducing the benzene content of the feedstock, characterized in that the method includes (a) at the first stage, contacting the combined feedstock comprising (I) a sulfur-containing cracking fraction naphthas boiling within the boiling range of gasoline, which includes paraffins, including low-octane n-paraffins, olefins and aromatic hydrocarbons, and (II) a benzene-containing fraction boiling within the boiling range of gasoline; under mild cracking conditions, at a temperature of from 204 to 427 ^{° C.} , with a solid acid catalyst consisting essentially of ZSM-5 zeolite with an average pore size having acid activity with an alpha value of from 20 to 200, for the alkylation of benzene with olefins with the formation of alkyl aromatic hydrocarbons and for cracking paraffins and olefins in the feed to obtain an intermediate product with a reduced benzene content relative to the combined feed; and (b) in the second stage, the contacting of the intermediate product with a hydrodesulfurization catalyst by combining elevated temperature, high pressure and a hydrogen-containing atmosphere to convert sulfur-containing compounds into an intermediate inorganic sulfur product and obtain a desulfurized product, which usually includes a liquid fraction with gasoline boiling limits. 2. The method according to claim 1, characterized in that the aforementioned fraction of naphtha cracking is a light fraction of naphtha with boiling limits in the range from C ₆ to 166 ^o C. 3. The method according to claim 1, characterized in that the above fraction of naphtha cracking is a wide fraction of naphtha with boiling limits in the range from ₅ to 216 ^° C. 4. The method according to claim 1, characterized in that the aforementioned naphtha cracking fraction is a heavy naphtha fraction with a boiling range in the range of 166 to 260 ^° C.  5. The method according to claim 1, characterized in that the above naphtha cracking fraction is an olefin fraction of catalytic naphtha cracking.  6. The method according to claim 1, characterized in that the benzene-containing fraction is a fraction of reforming products.  7. The method according to claim 1, characterized in that the hydrodesulfurization catalyst contains a metal of group VIII and group VI of the Periodic system of elements.  8. The method according to claim 1, characterized in that the first stage is carried out at a pressure of from 379 to 10446 kPa, hourly volumetric flow rate of the liquid is 0.5-10 and the ratio of hydrogen to hydrocarbon feedstock is 0-890 l / l (under normal conditions). 9. The method according to claim 1, characterized in that hydrodesulfurization is carried out at a temperature of 204-427 ^o C, a pressure of 379-10446 kPa, hourly volumetric flow rate of a liquid of 0.5-10 and a ratio of hydrogen to hydrocarbon feedstock 89-890 l / l ( under normal conditions). 10. A method of improving the quality of the sulfur-containing fraction of raw materials boiling within the boiling range of gasoline, which contains mononuclear aromatic hydrocarbons, including benzene, olefins and paraffins, and reducing the content of benzene in the fraction, characterized in that the method includes contacting the fraction in the first stage raw materials boiling within the boiling range of gasoline and containing mononuclear aromatic hydrocarbons, including benzene, olefins and low-octane paraffins, and a combined fraction including a sulfur-containing cracking fraction aphthae, and enriched with benzene reformate under mild cracking conditions at temperatures from 204 to 427 ^o C, with a solid acid catalyst having an average pore size consisting essentially of ZSM-5 zeolite having an acid activity to an alpha value from 20 up to 200, at a pressure of 2172-6998 kPa, a fluid hourly space velocity of 1-6 and a hydrogen to hydrocarbon ratio of 17.8-445 l / l (under normal conditions), for the alkylation of benzene with olefins to form alkyl aromatic hydrocarbons and for the cracking of olefins and low octane paraffins s in the feed, with conversion of olefins and naphthenes to aromatics is less than 25 wt%, benzene conversion from 10 to 60%, to form an intermediate product of reduced benzene content relative to feed.; hydrodesulfurization of the intermediate product in the presence of a hydrodesulfurization catalyst, at a temperature of 260-427 ^° C, a pressure of 2172-6998 kPa, a fluid hourly space velocity of 1-6, and a hydrogen to hydrocarbon ratio of 178 to 445 l / l (under normal conditions), to convert sulfur-containing compounds in the intermediate inorganic sulfur product and obtaining a desulfurized product with a total yield of liquid products of at least 90 vol.%.

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